(1) Field of the Invention
The present invention relates to an adhesive tape that is used in a working step, wherein a semiconductor wafer (for example, consisting of Si and GaAs), ceramics, glass, etc. is cut or abraded.
(2) Description of Prior Art
Hitherto, in semiconductor wafer dicing to make chips, a direct picking-up system has been used, wherein the semiconductor wafer is stuck and secured on an adhesive tape, the wafer is cut with a rotating circular blade along the chip shape, the securing adhesive tape is expanded radially to lower the adhesive force securing the chips, thereby enabling the chips to be picked up one by one from the adhesive tape, so that the chips can be mounted on dies.
In the above system, when the semiconductor wafer is cut with a rotating circular blade, for the purpose of cooling the rotating circular blade and to eliminate the cuttings, washing water pressurized to approximately 2 kg/cm.sup.2 is discharged against the rotating circular blade and the semiconductor wafer. Therefore, the chip-securing adhesive force in the dicing process is required to withstand the cutting impact force of the rotating circular blade and this water pressure, and in this sense, the greater the adhesive force the better. However, if the chip-securing adhesive force is excessive, the picking-up of the chips from the adhesive tape becomes difficult. That is, when it is required that the chip-securing adhesive force for the dicing process and the chip-securing adhesive force for the picking-up be controlled in accordance with the size of the chip, and the adhesive force of the adhesive tape set in accordance with the size of the chip, then, in the case of chips having a size of 25 mm.sup.2 or over, such as chips for recent LSI, whose integration degree has increased, the individual chip-securing adhesive force becomes high. As a result, a problem occurs in that the picking-up from the adhesive tape becomes difficult, and does not allowing a direct picking-up step to be applied.
To overcome this problem, adhesive tapes for securing a semiconductor wafer have been suggested that comprise a support through which radiation, such as ultraviolet rays or ionizing radiation such as an electron beam, can pass, and an adhesive layer that is applied on the support and is cured when irradiated, the chip-securing adhesive force in the dicing process is made high, and after the semiconductor wafer is cut and separated into chips, the tape is irradiated from the support side with radiation to cure the radiation-curable adhesive layer, thereby lowering the chip-securing adhesive force considerably, so that chips, regardless of size, such as of size of 25 mm.sup.2 or over, can be easily picked up.
Some adhesive tape suggestions have provide for semiconductor wafer-securing adhesive tapes having a radiation-transmitting support coated with a radiation-curable adhesive. These tapes are based on the principle that when the radiation-curable compound in the adhesive is cured it cause the adhesive to have a three-dimensional structure so that the fluidity of the adhesive is greatly lowered, and as a result the chip-securing adhesive force is lowered remarkably. Examples of such adhesive tapes are those disclosed in Japanese Patent Application (OPI) Nos. 196956/1985, 201642/1985, 28572/1986, and 10180/1987.
In the semiconductor wafer processing steps using a radiation-curable tape (e.g., sticking, cutting, grinding, polishing, radiation, and picking-up) such problems as described below may occur.
The surface state of semiconductor wafers that will be stuck on semiconductor wafer-securing adhesive tapes are not always specular or almost specular. That is, usually the wafer surface is subjected to a grinding treatment, etching treatment, or special treatment wherein a metal such as gold is deposited, and this results in a number of fine crevices in the wafer surface. Consequently, fluidization of the adhesive cannot be fully prevented by simply causing the adhesive layer to have a three-dimensional network after irradiation, and therefore the chip-securing adhesive force changes conspicuously depending on the surface state of the wafer. As a result, in some cases the picking-up of even chips having a size of 25 mm.sup.2 or below becomes difficult. Thus the change in the chip-securing adhesive force after irradiation due to the surface state of the chips (where sticking will be effected) leads to the problem that the picking-up workability lowers greatly, thereby lowering chip productivity.
Further, a film comprising a single resin layer, for example that of a polyvinyl chloride or ethylene-vinyl acetate copolymer is usually used as the substrate of a radiation-curable adhesive tape, but when it is desired to process the semiconductor wafer at a higher speed and more efficiently on an industrial production level, further problems mentioned below may arise.
When a semiconductor wafer or the like is diced using a radiation-curable adhesive tape, considerable tension is exerted on the adhesive tape because the support film is under the influence of an impact due to the cutting work by a rotating circular blade into the semiconductor wafer stuck on the adhesive tape, as well as being under the influence of the weight of the semiconductor wafer itself. Due to resulting tension, the support film is expanded, and sometimes the middle part of the adhesive tape may slacken and sink. When the thus-slackened adhesive tape is housed in a carrier cassette to be transported to the next step, such problems as the tape failing be suitably housed or the wafer-to-wafer contact in the carrier cassette can arise. Also, due to the slackening of the adhesive tape, the interval between the chips may becomes narrow, and the adjacent ends of the chips may touch, damaging the chips, which is another problem. Further, the radiation-curable adhesive tape will be subjected to irradiation to lower the chip-securing adhesive force as a pretreatment for picking up the chips in the irradiation process, wherein the adhesive tape is heated by light in the infrared range which falls on the adhesive tape. As a result, further expansion or partial wrinkling of the adhesive tape is caused by the heat or the like. Since this expansion or wrinkling will be retained until the final step where the radiation-curable adhesive is cured and the chips are picked up, the expansion and wrinkling not only present in the adhesive tape from being housed in a carrier cassette used for transportation between the steps, but also from irradiation, cause uneven intervals between the chips, leading to incomplete picking-up of the chips.
On the other hand, in such a system wherein the adhesive force is lowered by providing a three-dimensional network to the adhesive by the curing reaction of a radiation-curable compound, the rubber-like properties of the adhesive tape retained during the dicing step almost disappear when the picking-up operation is performed and therefore, a problem arises in that the radial expansion of the adhesive tape that has hitherto been performed, cannot now be done. This expansion of the tape, which will be carried out, or will not be carried out, depending on the performance of the wafer processing apparatus or the size of the chips, is carried out to prevent the adjacent chips from touching and damaging the circuit surface when the chips are picked up from the adhesive tape, because the interval between the chip cut by the rotating circular blade is too small. As a method of obviating this problem, an adhesive tape is suggested that will be irradiated after the tape has been expanded, as disclosed in Japanese Patent Application (OPI) No. 59684/1987. In that case, as a resin having rubber-like properties, is made of a resin such as polybutene, polyurethane, and 1,2-polybutadiene. Before irradiation these resins can be expanded to form an interval between the chips large enough to prevent the chips from touching, but if one tries to make the interval between the chips large enough to accommodate a picking-up apparatus that recognizes an image, so that the chips are picked up, the intervals between the chips become ununiform due to necking of the film itself, and also the film is deteriorated due to the irradiation or due to expanding after the irradiation, resulting in a disappearance of the rubber-like properties, so that such an enlargement of the interval between chips becomes workably impossible. The adhesive of this adhesive tape is identical with that disclosed in Japanese Patent Application (OPI) No. 19695/1985, and the expansion of the adhesive tape is to be performed before the irradiation. Generally, when expansion is carried out after irradiation, since the expanded tape is heated by the heat of the irradiation, there occurs the disadvantage that the chips are not arranged uniformly (the aligning property becomes poor). Further, in such a method, apparatuses for picking up chips that have been conventionally used by semiconductor manufacturers cannot be suitably used. (Generally an expanding mechanism is incorporated in a picking-up apparatus, and irradiation is performed before the step of expanding the adhesive tape.)